92 The need for power assisted steering

With manual steering a reduction in input effort on the steering wheel rim is achieved by lowering the

Dry road

Different^ in

'feel' due to surface

condition \

Wet road

Icy road


i > . _

Torque taction at steering wheel tfeell

Torque taction at steering wheel tfeell

Fig. 9.10 Typical relationship of tyre grip on various road surfaces and the torque reaction on the driver's steering wheel

Force at end of steering drop ari*rt (N)

Fig. 9.11 Comparison of manual steering with different reduction gear ratio and power assisted steering

Force at end of steering drop ari*rt (N)

Fig. 9.11 Comparison of manual steering with different reduction gear ratio and power assisted steering steering box gear ratio, but this has the side effect of increasing the number of steering wheel turns from lock so that manoeuvring of the steering will take longer, and accordingly the vehicle's safe cornering speed has to be reduced.

With the tendency for more weight to be put on the front steering wheels of front wheel drive cars and the utilization of radial ply tyres with greater tyre width, larger static turning torques are required. The driver's expectancy for faster driving and cornering makes power assisted steering desirable and in some cases essential if the driver's ability to handle the vehicle is to match its performance.

Power assistance when incorporated on passenger cars reduces the driver's input to something like 25-30% of the total work needed to manoeuvre it. With heavy trucks the hydraulic power (servo) assistance amounts to about 80-85% of the total steering effort. Consequently, a more direct steering box gear reduction can be used to provide a more precise steering response. The steering wheel movement from lock to lock will then be reduced approximately from 3 % to 4 turns down to about 2% to 3 turns for manual and power assistance steering arrangements respectively.

The amount of power assistance supplied to the steering linkage to the effort put in by the driver is normally restricted so that the driver experiences the tyres' interaction with the ground under the varying driving conditions (Fig. 9.10). As a result there is sufficient resistance transmitted back to the driver's steering wheel from the road wheels to enable the driver to sense or feel the steering input requirements needed effectively to steer the vehicle.

The effects of reducing the driver's input effort at the steering wheel with different steering gear overall gear ratios to overcome an output opposing resistance at the steering box drop arm is shown in Fig. 9.11. Also plotted with these manual steering gear ratios is a typical power assisted steering input effort curve operating over a similar working load output range. This power assisted effort curve shows that for very low road wheel resistance roughly up to 1000 N at the drop arm, the input effort of 10 to 20 N is practically all manual. It is this initial manual effort at the steering wheel which gives the driver his sense of feel or awareness of changes in resistance to steering under different road surface conditions, such as whether the ground is slippery or not.

9.2.1 External direct coupled power assisted steering power cylinder and control valve

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